Background: Small cell lung cancer (SCLC) affects ~50,000 people a year. A lack of effective treatment options offers many of these patients a less than 5% survival rate five years after diagnosis. Identifying ways to kill SCLC tumor cells is clearly an imperative. Like many tumors, SCLC tumors display recurrent copy number alterations (CNA) in specific parts of the genome. Studies of other tumor types have shown that discovering what recurrent genomic abnormalities drive tumor survival can lead to more effective treatment protocols and patient survival. Under this pretense two recurrent genomic changes in SCLC stand alone, a near universal mutation (~95%) of the retinoblastoma susceptibility gene (RB1) and genomic amplification of the Skp2 locus (~65%). Previous work demonstrated that in Rb-/- pituitary and thyroid tumors in mice, loss of Skp2 was synthetic lethal. Therefore, the combinations of mutations that contribute to SCLC development may be exploited to promote SCLC cell death. Objective/Hypothesis: I hypothesize that a significant portion of SCLC cells will be sensitive to Skp2 targeted therapy. My preliminary studies support this notion. I propose to define the molecular profile of a Skp2- sensitive SCLC cell and to identify the mechanism by which loss of Skp2 initiates apoptosis in these cells. Specific Aims: (1.) To determine which SCLC cell lines require Skp2 for proliferation and survival. (2.) To identify the mechanism of synthetic lethality between loss of pRB and Skp2 in tumor cells. (3.) To elucidate the compensatory survival pathways in SCLC cells that promote viability following loss of Skp2. Study Design: Cell viability assays will be done in a panel of 58 SCLC cell lines housed at Center for Molecular Therapeutics at the MGH Cancer Center following lentiviral shRNA knockdown of Skp2 or a Scramble shRNA control. Gene expression profiles will be used to identify signatures that distinguish Skp2- dependent lines, from Skp2-independent SCLC cells. Gene expression profiles will be compiled following Skp2 knockdown in lines both sensitive and resistant to loss of Skp2. Previous work has suggested that loss of Skp2 induces apoptosis through a mechanism involving p27 function; alternatively, other work has shown that previous work demonstrated that loss of Skp2 promotes ER-stress. High levels of ER-stress are known to lead to apoptosis. Therefore the role of p27 and the ER-stress response will be assessed in the context of the synthetic lethality following loss of Skp2 in SCLC cells. The gene expression profiles will provide a basis for understanding why some cells and not others are sensitive to loss of Skp2. For cells tolerant of Skp2 loss alone, assessment of response to cellular stress will be determined in the presence and absence of Skp2. This will lead to identification of combined approaches that together with Skp2 knockdown will lead to SCLC cell death.